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Linux
To query for a PTR record of some specific IP, you first reverse the IP address into the canonical form, then query for a record as usual. The canonical form is 194.90.1.5 -> 5.1.90.194.in-addr.arpa . Few examples:

$dig +short PTR 8.8.8.8.in-addr.arpa

google-public-dns-a.google.com.

$dig +short PTR 5.1.90.194.in-addr.arpa

dns.netvision.net.il.
DIG also supports shorter form of PTR query using -x switch: